The use of x-rays for analyzing the physical and chemical characteristics of solids, liquids, and gases is generally known. Specific techniques utilizing x-rays include: x-ray absorptiometry, x-ray emission also called x-ray fluorescence, x-ray photoelectron spectroscopy, extended x-ray fine structure absorption spectroscopy, x-ray lithography, and x-ray microscopy. X-rays for use in these techniques may be obtained from: (a) bombardment of a solid with charged particles (electrons or ions); (b) radioactive decay (electron capture, internal conversion); or (c) high energy electrons in circular motion (synchrotron radiation).
A conventional device for producing x-rays is an x-ray tube in which a beam of electrons, each having an energy level in the range of approximately 1-10 keV, is accelerated into a metal anode. The metal anode is typically a metal foil or metal disk. The interaction of these energetic electrons with the metal anode results in the emission of photons with frequencies ranging from visible frequencies through x-ray frequencies. The energy of a photon is proportional to its frequency. The emitted photons may be classified as "bremsstrahlung" or characteristic x-rays according to their energy distribution. The photons are bremsstrahlung when they have frequencies continuously distributed across a range. Bremsstrahlung photons are generated as the electrons lose kinetic energy in the metal anode. The tube anode material, the operating voltage of the tube, and the electron current through the tube all help to determine the nature of bremsstrahlung photons generated by the x-ray tube.
In addition to bremsstrahlung x-rays, characteristic x-rays of the anode material may also be generated by the x-ray tube. These characteristic x-rays are produced at certain frequencies dependent upon the anode material because they are generated by the atomic de-excitation of the anode atoms. The output of an x-ray tube is the summation of the bremsstrahlung and characteristic x-ray distributions. The x-ray tube is a copious source of photons. However, the intense output is achieved at the cost of relative system complexity, size and mobility.
In contrast, a radioactive x-ray source is a simple device, and more compact than the x-ray tube. However, the radioactive x-ray source generally produces a less intense output than the x-ray tube. Additionally, while an x-ray tube can be turned off at will, the radioactive x-ray source cannot be turned off at will. The lack of control in the output of radioactive sources presents radiological safety problems.
Conventional x-ray tubes generally require high power in order to operate properly. This high power can be from watts to kilowatts in aggregate. Further, due to high power dissipation in the form of heat, cooling is generally required. The cooling system in the x-ray tube also adds size, weight, cost, and complexity.
Accordingly, a need has arisen for an x-ray source which is compact, but able to provide relatively high x-ray intensity.